Phytochemicals are chemical compounds that occur naturally in plants and are among other things, responsible for color such as exemplified by the deep purple of blueberries and organoleptic properties such as exemplified by the smell of garlic. Some phytochemicals are used in nutraceutical products that are generally sold in medicinal forms not usually associated with food.
There are three classes of phytochemicals that are of particular interest i.e., polyphenols, specialty carbohydrates, and glycosides. Polyphenols, also referred to as phenolics, are compounds that function mainly as antioxidants and anti-inflammatories when ingested by humans. An antioxidant is a molecule that inhibits the oxidation of other molecules. Oxidation in living cells can cause damage or death to the cell. Antioxidants prevent this damage by being oxidized themselves, instead of the cell components. Antioxidants are widely used in dietary supplements and have been investigated for the prevention of diseases exemplified by cancer, coronary heart disease, altitude sickness, among others. They are also used as preservatives in food and cosmetics. As antioxidants are present in food consumed in human diets and in plants used in traditional medicine of several cultures, their roles in human health and disease are subjects of much research. Polyphenols can be synthesized industrially, but they are mainly made available from plants and microorganisms.
Carbohydrates are saccharides that perform numerous roles in living organisms. Carbohydrates serve as the body's source of energy (e.g., starch and glycogen), and as structural components (e.g., cellulose in plants and chitin in fungi and arthropods). Short-chain carbohydrates are also called sugars, while long-chain or complex carbohydrates are known as polysaccharides or oligosaccharides. Carbohydrates and other compounds derived from them can play key roles in mammalian immune systems, fertilization, preventing disease or infection, blood clotting, among others.
A sugar bound to another functional molecule (e.g., a sugar bonded to a phenolic) is known as a glycoside. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. These can be activated by a hydrolysis reaction, which causes the sugar part to be broken off, making the chemical available for use. Many such plant glycosides are used as medications.
The current approach to the extraction of plant components is through use of either organic solvents or unpressurized hot water to solubilise and remove these components from plant biomass. The organic solvent systems commonly use one or more of ethanol, methanol, ethyl acetate and acetone. However, organic solvents are generally toxic and their commercial use requires explosion-proof facilities provided with storage and handling equipment certified for use with toxic and flammable chemicals. Furthermore, solvents may remain in final products as unhealthy trace compounds and their toxic properties raise safety concerns for human consumption.
It is well-known that hot-water systems tend to be less efficient than organic solvent-based systems and are able to only extract a portion of the potentially available phytochemicals from plant biomass.
In addition to nutraceuticals, biomass can be a valuable source of chemical products. Lignocellulosic biomass is one of the most abundant materials in the world and considerable attention has been given to its use as a raw material for the production of energy and chemicals. Fractionation of lignocellulosic biomass to improve utilization of its constituent components of cellulose, hemicellulose, and lignin can be accomplished using various physical, biological, thermal, or chemical methods. Hydrothermal treatments (also known as autohydrolysis, hydrothermolysis) include steam explosion, pressurized low polarity water (PLPW; also commonly referred to as superheated water, subcritical water, pressurized hot water, compressed hot water), which uses the catalytic action of hydronium ions from water ionization due to the processing conditions, and the production of in situ acids (such as acetic acid generated from acetyl groups), to hydrolyse the carbohydrates within the biomass. Heating water under pressure to temperatures above its boiling point results in alteration of its key properties such as pH and polarity and decreases its dielectric constant to values that approximate those of solvents such as those exemplified by ethanol and methanol.
Batch processing and continuous flow-through systems using hydrothermal water treatments have used to process, in very small-volume systems, a wide range of lignocellulosic feed stocks including hardwood chips from eucalyptus, poplar, Luecaena sp., maple, sweet gum, vegetative material and straws from annual plants including wheat straw, barley straw, rye straw, oat straw, Brassica sp. straws, flax shives, sorghum, switch grass, sugarcane among others. It is known that product yields from flow-through hydrothermal treatments are vastly different from those produced with batch systems. Flow-through reactors have been shown to remove more hemicellulose and lignin, with fewer degradation products forming than in a batch system. Nearly complete hemicellulose removal is possible with flow-through systems, whereas only 60% removal has been achieved in batch systems (Lui et al., 2003, The Effect of Flow Rate of Compressed Hot Water on Xylan, Lignin, and Total Mass Removal from Corn Stover. Ind. Eng. Chem. Res. 42:5409-5416). Furthermore, lignin removal is less than 30% in batch reactors, but up to 75% lignin removal is possible in flow-through systems at high flow rates (Lui et al., 2003). Additionally, hemicelluloses in flow-through reactors are recovered mostly as oligosaccharides (Lui et al., 2003).
However, successful scale-up of the small laboratory systems to large throughput commercial volume systems has not yet been achieved because of the problems associated with the attaining and maintenance of high pressures in large extraction vessels to provide constant pressures and temperatures while maintaining a constant throughput of feedstock materials. Problems commonly encountered in such scale-up attempts include material agglomeration, development of fluid channelling, blockages in feedstock material throughputs, and back mixing resulting in heterogeneous extractions and significantly reduced extraction efficiencies when compared to the results achieved with small laboratory-scale equipment.